In recent years, secondary batteries, which can be charged and discharged, have been widely used as an energy source for wireless mobile devices. In addition, the secondary batteries have attracted considerable attention as an energy source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a middle or large-sized battery module including a plurality of battery cells electrically connected to each other because high output and large capacity are necessary for the middle or large-sized devices.
Preferably, the middle or large-sized battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle or large-sized battery module. In particular, much interest is currently focused on the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the weight of the pouch-shaped battery is small, and the manufacturing cost of the pouch-shaped battery is low.
However, the mechanical strength of the sheathing member is low. For this reason, battery cells (unit cells) are mounted in a pack case, such as a cartridge, so as to manufacture a battery module having a stable structure. However, a device or a vehicle, in which a middle or large-sized battery module is installed, has a limited installation space. Consequently, in a case in which the size of the battery module is increased due to the use of the pack case, such as the cartridge, the spatial utilization is lowered. In addition, the battery cells repeatedly expand and contract during charge and discharge of the battery cells due to the low mechanical strength of the battery cells. As a result, thermally welded regions of the battery cells may be easily separated from each other.
In order to solve the above problem, therefore, Korean Patent Application No. 2006-45444, which has been filed in the name of the applicant of the present patent application, discloses a battery module including plate-shaped battery cells, each of which has electrode terminals formed at the upper end and the lower end thereof, wherein the battery module includes two or more battery cells, electrode terminals of which are connected in series to each other, connection parts between the electrode terminals being bent such that the battery cells are stacked, and a pair of high-strength cell covers coupled to each other for covering the entirety of the outer surface of the battery cell stack.
In the above disclosure, during assembly of the battery module, two pouch-shaped battery cells are arranged in series in a longitudinal direction such that the electrode terminals of the battery cells are continuously adjacent to each other, and then the electrode terminals of the battery cells are coupled to each other by ultrasonic welding. Subsequently, the two battery cells, the electrode terminals of which are coupled to each other, are folded such that the battery cells face each other to form a battery cell stack, the battery cell stack is mounted in the cell covers to manufacture a unit battery module, and electrode terminals between unit battery modules are connected to each other to manufacture a middle or large-sized battery module.
As previously described, however, during connection of the electrode terminals between the unit battery modules or connection of the electrode terminals and electrode tabs, the electrode terminals and the electrode tabs are welded to each other by ultrasonic welding before the battery module is mounted in a pack housing, and the battery module is mounted in the pack housing after the connection of the electrode terminals. Consequently, an assembly process is complicated and, in addition, impact generated during the assembly process may be transmitted to the electrode terminals with the result that an internal short circuit may occur.
In addition, the positive electrode terminal and the negative electrode terminal of the battery module are made of different materials. For this reason, welding coupling force is low when the positive electrode terminal and the negative electrode terminal are welded to each other. Furthermore, when external force, such as vibration and impact, is applied to the middle or large-sized battery module manufactured as described above, the welded regions of the battery module may be separated from each other with the result that a short circuit may occur.
Therefore, there is a high necessity for technology that is capable of easily assembling a middle or large-sized battery pack and improving welding reliability while solving the above problems, thereby fundamentally securing safety of the battery pack.